FREQUENCY_DETECTING_COMPARATOR

Published:2009/7/6 21:30:00 Author:May | From:SeekIC

A quad comparator forms the basis of a frequency detector that is faster and less expensive than more complex versions designed around frequency-to-voltage converter chips. Positive feedback through a 5-MΩ resistor allows the circuit to resolve changes as small as two percent; the output responds to those changes in about one cycle. When the input frequency is high, V2 is pulled low; it's never allowed to exceed 2/3 V. When the input frequency is lower than the limit, V2 exceeds 2/3 V once each cycle, but V3 is held below that limit. The trip frequency is defined by F = 1/(1.1R2C2). R2 can be adjusted to permit trimming of the trip point, but the value of R3 must remain larger than R2.



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